The present invention relates to a system for deciding the combustion state of each cylinder in an internal combustion engine, and a fuel control system which optimizes a fuel injection quantity while suppressing the combustion change of each cylinder after starting of engine and reduces a non-combustion composition in an engine exhaust gas.
Generally, a multi-cylinder engine having a fuel injection system has different combustion states due to different injection characteristics of fuel injection valves and different intake air distributions for the respective cylinders.
Particularly, when a cooled engine is started, in order to compensate for the attenuation of the vaporizing characteristic of fuel, a fuel injection quantity is increased according to the temperature of engine coolant. The quantity of fuel to be increased in starting of engine is set for a prescribed value for all cylinders relative to the cylinder having the poorest fuel contribution.
Therefore, a large quantity of incomplete combustive fuel is exhausted from a cylinder to which excessive fuel has been supplied when the engine is started, thus giving rise to a problem of air pollution.
In order to solve such a problem, it is necessary to control the distribution of fuel to be injected for each cylinder to supply an optimum quantity of injection fuel to each cylinder so that the combustion states of the respective cylinders are averaged and the fuel injection quantity set according to a coolant temperature and others is reduced within a range not deteriorating the combustion state.
In order to detect fuel distributed properly, means for directly measuring the combustion state of each cylinder is required. As an example thereof, a technique using an ion current is disclosed in JP-A-7-293306.
Such a combustion control technique for each cylinder (also referred to as cylinder-individual combustion control technique) is to control fuel for each cylinder on the basis of the comparison of an ion current output maximum value and an integrated value of each cylinder with a reference value so as to reduce the fuel injection quantity for each cylinder.
The above conventional cylinder-individual combustion control technique controls the fuel injection quantity for each cylinder by reducing a difference in the combustion state among the respective cylinders. Therefore, it can suppress engine vibration due to a difference in the combustion state among the respective cylinders. But it does not necessarily reduce the fuel injection quantity for all the cylinders and hence does not perform an optimum control.
Further, the above conventional cylinder-individual combustion control technique decides the combustion state on the basis of the maximum value and integrated value of the ion current acquired from the combustion state in a present cycle of each cylinder. However, the combustion state of each cylinder varies for each cycle. Therefore, the conventional control technique cannot provide a correct value of the combustion state only from the combustion state in the present cycle, thus making it impossible to make appropriate decision.